I would assume you go into Zombies, and at the level select, press down. It should light up.
I haven't gotten it, though, so I don't know. But there is a part of the map screen that's inaccessible yet slightly lit-up, and I'm assuming that's Nuketown. --- Currently playing: Rhythm Thief and the Emperor's Treasure - Let's Play on my YouTube! :D My Gaming YouTube: http://www.youtube.com/user/MasterChief3624